Original Date: 04/24/2007
Revision Date: / /

Laser Holographic Hole Drilling research conducted at the University of New Orleans, College of Engineering has the potential of making residual stress analysis measurements an inexpensive, quick, and highly accurate process for industry.

The shipbuilding industry is beginning to implement new technologies such as automated welding, cutting, and material movement in shipbuilding processes. In order for these technologies to work as envisioned, the shipyards will have tighter dimensional control of parts and must develop increased control of distortion of parts. The distortion of steel plates is almost always due to the residual stress (RS) that is caused by manufacturing processes. Some of those processes are milling, drilling, cutting, grinding, and welding – all necessary steps in the manufacturing of ships. Since the elimination of RS is not possible using today’s manufacturing process, an inexpensive and quick method of measuring the stress is necessary. Once the degree of stress can be identified and measured inexpensively and corrective actions can be taken to reduce the stress, distortion can be reduced before other manufacturing processes are impacted.

Traditional methods of measuring stress are Strain Gage Hole Drilling (SGHD) and X-ray Diffraction (XRD). Other methods are available, including Synchrotron and Neutron Diffraction, but neither is considered to have a high degree of reliability. The University of New Orleans, College of Engineering (UNO COE) has recently undertaken a project to assess the capabilities of a new technology for measuring residual stress in materials. This technology is called Laser Holographic Hole Drilling (LHHD). In the mid-1980s, research found that holographic hole drilling caused an interference fringe pattern related to the displacements that occurred as a result of the hole drilling to the subsurface residual stress. In-plane sensitive electronic speckle pattern interferometry with automated fringe analysis was developed in 2000 for rapid stress analysis. The attainment of real-time or near-real-time results of the tests are now available. One RS measurement typically takes only five minutes.

The research team at the UNO COE measured RS in test specimens using several techniques, including SGHD, XRD, and LHHD. SGHD and XRD were selected because they are the industry standards. Each of the three methods for measuring RS has advantages and disadvantages. LHHD may become the preferred method of measuring RS in the shipbuilding industry. The advantages of LHHD over other methods include portability and quickness, no surface preparation, no costly strain gages, automated drilling and data analysis, and only two material properties required (Young’s Modulus and Poisson’s Ratio). LHHD is also semidestructive and does require hole drilling. A comparison of the results of the three technologies using identical base material conditions indicated that LHHD can become the most effective test method for determination of RS. However, the vibrations always present in a production atmosphere make LHHD measures problematic. This issue must be resolved before LHHD replaces SGHD or XRD as the preferred method of residual stress measurement in an industrial setting.

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